Carbon face seals are used in machinery, such as turbine engines, to effect a fluid seal between regions of high and low fluid pressure. For example, carbon seals are used to prevent hot, high pressure air from entering a bearing compartment operating at a lower pressure. A typical carbon seal for a turbine engine includes an annular carbon ring secured to an annular, nonrotatable, axially translatable seal housing. The seal also includes a seal seat affixed to a rotatable shaft and positioned axially adjacent to the carbon ring. The carbon ring comprises a base (or blank) and a nose projecting axially from the base. The nose is urged into contact with the seal seat by a combination of spring forces acting on the seal housing and the net resultant of axially opposing fluid pressure forces acting on the seal housing and the carbon ring. The contact area between the carbon ring and the seal seat equals the annular area of the nose. The contact between the nose and the seal seat resists fluid leakage across the seal in the radial direction, i.e. toward or away from the axis of rotation of the seal seat.
During operation, the nose gradually wears away. Ordinarily, the seal is replaced or refurbished before the nose is completely worn away. Occasionally, however, accelerated seal wear can result in complete wear of the nose so that the base of the carbon ring contacts the seal seat. As a result, the contact area between the carbon ring and the seal seat equals the annular area of the base, which is larger than the contact area of the nose. This affects the resultant of the axially opposing fluid pressure forces such that the net pressure force is less favorable for maintaining reliable, positive contact between the carbon ring and the seal seat. Unfortunately, the transition between the normal condition in which the nose contacts the seal seat, and the highly deteriorated condition in which the base contacts the seal seat, although it occurs very infrequently, can occur with little warning. In addition, more abrupt failure or deterioration of the carbon ring can have a similar adverse effect on the resultant of the fluid pressure forces. As a result there may be an unanticipated period of engine operation during which fluid leaks past the seal.
What is needed is a carbon seal that deteriorates gracefully in order to exhibit a detectable and benign operating characteristic that clearly indicates that maintenance is required.